Shrink tunnel for applying shrink films, method for operating or controlling a shrink tunnel, and production system having a shrink tunnel

ABSTRACT

An apparatus comprises a shrink tunnel for use in a production system for the production of packaging units from packaging materials filled with a product and having shrink film shrunk thereon, the shrink tunnel comprising a tunnel zone, heating means for application of heat to shrink the shrink film onto packaging units moved through the tunnel zone, the tunnel zone being maintained at a tunnel temperature equal to a target operating temperature, wherein the tunnel has an operating mode and a standby mode, wherein heat output during the standby mode is less than heat output during the operating mode, and wherein the standby mode is triggered by at least one of a time control and a signal control.

The invention relates to a shrink tunnel for applying shrink films on packaging units or containers in accordance with the preamble to patent claim 1, to a method for operating or controlling a shrink tunnel in accordance with the preamble to patent claim 10 and to a production system with a shrink tunnel in accordance with preamble 19.

Known in production systems, in particular in those in the drinks industry, is the feeding of packaging materials, for example bottles, cans or other packaging materials, filled with a product, sealed and labelled in prior system components or machines, to a subsequent packaging machine in which the packaging materials for example are assembled on packaging elements into packaging material groups with a set number of packaging materials and the packaging material groups thus produced are then covered with a shrink film. In a shrink tunnel after the packaging machines, a firm or fixed packing unit is formed from each packaging material group by shrinking on the shrink film under the effect of heat, in particular by blowing with hot air.

To ensure shrink-wrapping of the necessary quality, it is usual in known systems to operate the shrink tunnel which generally has a number of consecutive heating or tunnel zones each with its own heating system in the direction of transport of the packaging units or containers, so that all the tunnel zones are constantly kept at the target operating temperature required to shrink on the shrink film, for example a temperature in the range between 195° C. and 210° C., preferably around 200° C., even during operating interruptions during which no packaging materials or packaging material groups are moved through the shrink tunnel for the shrinking-on of the shrinking film, caused for example by malfunctions in the system components or machines before or after the shrink tunnel. The heating means are generally electric or gas-powered heating elements, in particular hot-air blowers too.

Although with operating interruptions of this kind due to the absence of packaging materials or packaging material groups being moved through the shrink tunnel which reduces the heat output needed to maintain the tunnel temperature or target operating temperature, for example to around 40% to 45% of the heat output needed in undisturbed normal operation of the system, this operating method of known shrink tunnels results in a considerable energy expenditure with considerable energy or heating costs and the associated unnecessarily high waste gas and CO2 emissions.

The purpose of the invention is to describe a shrink tunnel which avoids these drawbacks. To resolve this task, a shrink tunnel is developed in accordance with patent claim 1. A method for operating or controlling a shrink tunnel is the subject of patent claim 10. A production system for the production of packaging units or containers with these enclosing and/or fixing shrink films is the subject of patent claim 19.

In a further development of the invention, the shrink tunnel and/or the method for its control are beneficially developed such

-   -   that the tunnel temperature in the shrink tunnel and/or in the         at least one tunnel zone is set or controlled above the output         of the heating means generating the tunnel temperature such that         during the normal operating mode the tunnel temperature reaches         the target operating temperature, for example a target operating         temperature in the range between 190 and 210° C., preferably a         target operating temperature of 200° C. and in standby mode the         tunnel temperature in at least one tunnel zone is lowered by a         temperature amount, for example by a temperature amount of         50° C. to 80° C. from the target operating temperature to a         reduced tunnel temperature.     -   and/or     -   that in several of the consecutive tunnel zones in the direction         of transport, to which in each case specific heating means are         allocated, they are controlled or set independently for         switching the operating modes, this being preferably depending         on the control or adjustment parameters set or stored in the         control unit,     -   and/or     -   that the target operating temperature and/or the reduced tunnel         temperature are adjustable and/or are stored in the control unit         as control or adjustment parameters,     -   and/or     -   that the heating means of the at least one tunnel zone (14) are         controlled or set between a heat output corresponding to a base         load and a maximum heat output, for example between a heat         output (base load) of 10 KW to 14 KW and a heat output of 46         KW-50 KW,     -   and/or     -   that the standby mode in the presence of at least one signal         indicating an operating interruption, for example a signal         delivered by a superordinate control unit of the production         system (line signal) and/or triggered with a time delay, for         example with a time delay in the range of 1 to 2 minutes,     -   and/or     -   that the return of the shrink tunnel to normal operating mode         occurs after the disappearance of the error message, in         particular by increasing the heat output or the increase of the         tunnel temperature to the target operating temperature,     -   and/or     -   that after the triggering of the standby mode, a reduction in         the transport speed of a conveyor on the shrink tunnel side, for         example a reduction in the transport speed to 20% to 40% of the         transport speed of the normal operating mode and/or the         switching-off of a cooling system in the transport element on         the shrink tunnel side and/or a cooling system which serves to         cool the packaging units after the shrinking-on of the shrink         film,     -   and/or     -   a switching-off of the blowers of the heating means occurs,     -   and/or     -   that when an error message disappears or the shrink tunnel is         reset to normal operating mode, the tunnel temperature is reset         to the target operating temperature,     -   and/or     -   that when an error message disappears or the shrink tunnel is         reset to normal operating mode, the blowers of the heating means         are switched on,     -   and/or     -   that when an error message disappears or the shrink tunnel is         reset to normal operating mode and when a first temperature         threshold of the tunnel temperature below the target operating         temperature is reached, the transport speed of the conveyor on         the shrink tunnel side (13) is increased again, this being for         example at a first temperature threshold lying around 5% below         the temperature threshold,     -   and/or     -   that when an error message disappears or the shrink tunnel is         reset to normal operating mode and when a first temperature         threshold of the tunnel temperature below the target operating         temperature is reached, a cooling system of this transport         system is switched on, for example at a first temperature         threshold lying around 5% below the temperature threshold,     -   and/or     -   that when an error message disappears or the shrink tunnel is         reset to normal operating mode and when a second threshold         closer to the target operating temperature is reached, lying for         example at 2% below the target operating temperature, a signal         triggering production approval is generated and/or the cooling         system for the packaging units or containers (4) is switched on,         whereby the aforesaid characteristics can be used individually         or in any combination.

Further developments, benefits and application possibilities of the invention arise from the following description of examples of embodiments and from the figures. In this regard, all characteristics described and/or illustrated individually or in any combination are categorically the subject of the invention, regardless of their inclusion in the claims or reference to them. The content of the claims is also an integral part of the description.

The invention is explained in more detail below using the FIGURE which shows in a simplified functional representation a system section 1 of a production system for packaging material 2 filled with a product, for example for packaging material filled with a liquid substance, e.g. with a drink. The system section of the production system generally labelled 1 and illustrated in the FIGURE serves specially for the assembly of the packaging material 2 filled with the product into packaging material groups 3 and for the packaging of each packaging material group 3, inter alia using a shrink film, so that the packaging material groups 3 in each case after the shrinking-on of the shrink film form packaging units or containers 4 each with a specified number of packaging materials 2.

The packaging materials 2 are for example bottles, cans or similar containers which are filled with the product, sealed and labelled. System section 1 has for this in a transport direction A, in which the packaging materials 2 or packaging material groups 3 and finally also the packaging units or containers 4 are moved, consecutively inter alia the following system components with the following functions:

-   -   A buffer or storage unit 5, which for example is formed by a         buffer or storage table and which serves for the interim storage         of the packaging materials 2 fed to this device via an external         conveyor 6 in transport direction A.     -   A packaging machine 7 in which the packaging material groups 3         are formed from the packaging materials 2, in the embodiment         shown this being on or in a packaging element 8, for example in         the form of a carton-type packaging element or plate or tray         such that each packaging material group 3 has packaging         materials 2 in a specified number and arrangement to each other,         for example in the illustrated embodiment in two rows each of         three packaging materials 2. In the packaging machine 7, each         packaging material group 3 including its packaging element 8 is         covered with the shrink film, preferably such that the         overlapping ends of this covering are on the underside of the         packaging material group 3 or packaging element 8.     -   The packaging elements 8 or sections forming them are fed into         the packaging machine 7 via a conveyor 9 (arrow B).     -   A shrink tunnel 10, in which the shrink film is shrunk onto the         various packaging material groups 3 to form packaging units or         containers 4, this being under the effect of heat, in particular         under the effect of hot air on the product groups 3 covered with         the shrink film and moved through the shrink tunnel 10 in         transport direction A.

After leaving the shrink tunnel 10, the packaging units or containers 4 are fed via an external conveyor 11 to a further use or treatment, for example a device formed as a pallet-loader.

Within system section 1, the buffer or storage unit 5, the packaging machine 7 and the shrink tunnel 10 are linked to each other by a transport system 12 formed by at least one conveyor or conveyor belt, whereby the shrink tunnel 10 however has an independent and independently controllable transport system for the packaging material groups 3 or the packaging units or containers 4 formed from them in the form of at least one endlessly running drivable conveyor belt 13 (e.g. wire link conveyor or flat-top chain conveyor).

As with packaging machine 7, the shrink tunnel 10 is also for example of the normal design for these system components. Thus, the shrink tunnel in the illustrated embodiment is for example formed with a number of consecutive heating or tunnel zones 14 in transport direction A, for example with a total of three tunnel zones 14, thereby ensuring inter alia that in normal undisturbed operation of the system or the shrink tunnel 10, even with a close succession of packaging material groups 3 moved through the shrink tunnel 10 in transport direction A, each packaging material group 3 has shrink film shrunk on to the necessary quality. Each tunnel zone 14 is, in the illustrated embodiment, equipped with its own heating means shown only very schematically in the FIGURE by 15, which under the effect of heat causes the shrinking of the shrink film onto the packaging material groups 3 and includes at least in each case one hot-air generator comprising at least one blower and at least one heating installation allocated to the blower and if applicable at least one additional heating installation for a basic heat output. To prevent energy losses, lamellar curtains 16 are provided at the entrance and exit of the shrink tunnel 10.

Shrink tunnel 10 also includes a cooling installation not illustrated (e.g. cooling air blower), with which the at least one conveyor belt 12 is cooled along a conveyor belt length outside the shrink tunnel 10 and in the direction of circulation before the entrance to the shrink tunnel 10, so that each packaging material group 3 fed into the shrink tunnel 10 and covered with shrink film is first passed onto the cooled conveyor belt 13. The shrink tunnel 10 also has, in transport direction A and following the tunnel zones 14, a cooling zone 17 indicated in the FIGURE by dashed lines, which serves inter alia to cool the packaging units or containers 4 and to this end is fitted for example with a cooling air blower.

In undisturbed operating status of the system, the shrink tunnel 10 is operated in its normal operating status or operating mode, which is characterised inter alia by the following operating parameters:

-   -   Shrink tunnel 10 or its tunnel zones 14 are at target operating         temperature which is sufficient for the shrinking-on of the         shrink-film to the required quality. This target operating         temperature which is the tunnel temperature inside the shrink         tunnel 10 or inside the tunnel zones 14 and thus in particular         at least the temperature of the hot air emitted into the tunnel         zones 14, is for example 200° C.     -   The at least one conveyor belt 13 is driven at production speed,         i.e. at a speed which matches the system capacity and thus the         capacity of packaging machine 7 and the transport speed of the         transport system 12 and in general is well above 8 m/min.     -   The heating means 15 are controlled or set to maintain the         target operating temperature between a basic heat output which         is for example 10-15 KW for each tunnel zone 14, and a maximum         heat output or total heat output which is for example 45-50 KW         for each tunnel zone 14.     -   The cooling installation 17 or the blower in it generating         cooling air is switched on.

A particular feature of the system or the shrink tunnel 10 consists of the fact that in the event of operating interruptions occurring in the system, the shrink tunnel 10 or its tunnel zones 14 go into a standby operating status or standby mode with a clearly reduced performance compared to the normal undisturbed operating status and return automatically to normal operating status once the operating interruption has been remedied. Operating interruptions of this kind which trigger standby mode are for example a number of packaging materials 2 below a specified quantity of packaging materials in the buffer and storage unit 5 and/or a packaging material jam inside the system, e.g. in a section of the system before and/or after the shrink tunnel 10 based on the direction of transport A. The shrink tunnel 10 is controlled automatically between the two operating statuses, i.e. in the illustrated embodiment by an electronic control unit 18, which is for example a smart control unit allocated to the shrink tunnel 10 which also adjusts or controls the heating means 15 to maintain the necessary tunnel temperature at any time. The control unit 18, which is connected for example via a bus system, not illustrated, to further control units or to a central or overriding system control, controls the switching between the operating statuses of the shrink tunnel 10 (normal operating mode/standby mode) depending on error messages for example from the overriding system control and/or depending on other signals. Preferably, in the control unit 18 for the detection of existing operating interruptions, at least partially signals are processed which serve to control other system components, for example to control the packaging machine 7. These signals are, for example, “Insufficient material in buffer or storage unit 5” and/or “Insufficient packaging materials in packaging machine 7”, and/or “Minimum product” as examples of error statuses in direction of transport A before the shrink tunnel 10, and/or “External bottleneck” and/or “No pallets” as error statuses in direction of transport A after the shrink tunnel 10 etc.

With the standby mode, the following measures inter alia are triggered which can each be carried out singly or in any combination:

-   -   Lowering of the tunnel temperature by a specified amount ΔT, for         example by 50° C. to 80° C., preferably by around 50° C. below         the target operating temperature, whereby this lowering of the         temperature or the amount ΔT actually at the shrink tunnel 10 or         at an input to the control unit 18 can be set to adapt to the         concrete production conditions, for example also for each tunnel         zone 14 individually.     -   Switching-off of the base load, i.e. the heating elements         producing the basic heat output, which is primarily to heat the         shrink tunnel 10 with electrical energy, whereas if the shrink         tunnel 10 is gas-powered, for safety reasons alone the tunnel         zones 10 are heated only with hot air. Where the tunnel         temperature is lowered by the amount ΔT, there is necessarily a         corresponding drop in the base heat output in a gas-powered         shrink tunnel 10, i.e. the heating elements or gas burners         adjust automatically to the output needed at any time and in         some cases switch off briefly.     -   Reduction in the transport speed of the transport system on the         shrink tunnel side or of the at least one conveyor belt 13 to a         transport speed which is a mere 20% to 40% of the transport         speed in the normal operating status of the shrink tunnel 10.     -   Switching-off of the cooling system in the cooling zone 17 to         cool the packaging units or containers 4 and the at least one         conveyor belt 13.     -   Switching-off of one blower per tunnel zone 14, provided that at         least two electrically driven blowers are allocated to each         tunnel zone 14, as is generally the case.

The standby mode, which preferably can also be triggered in situ, i.e. at the shrink tunnel 10 or at the control unit 18, is triggered with a time delay At1. The length of this time delay At1 is preferably selected so that it is long enough for the packaging material groups 3 which at the time of receipt of at least one error message causing the transition into standby mode, are already in the shrink tunnel 10 and possibly also in a section of the system before this shrink tunnel 10 based on direction of transport A, for example in the packaging machine 7 and/or on the transport system 12 between the packaging machine 7 and the shrink tunnel 10, to be handled dependably and with a high level of operational reliability in the shrink tunnel 10, i.e. the shrink film is still shrunk correctly onto these packaging material groups 2 too. The time delay At1, at which the standby mode is triggered, is for example 1 to 2 minutes.

The return of the shrink tunnel 10 from standby mode to normal operating status or mode occurs once the error message(s) has/have gone and also after a delay due to the time needed for re-heating to the target operating temperature, i.e. with a time delay At2 for example of 2 to 3 minutes. Only after this time delay is the packaging machine 7 also turned back on again, which of course was switched off or in standby operation during the standby mode of the shrink tunnel 10. Preferably, the packaging machine 7 and the transport system 12 conveying the packaging material groups 3 to the shrink tunnel 10 are switched back on only once the tunnel temperature has reached the target temperature or at least a temperature very close to the target operating temperature. The packaging machine 7 and preferably also the transport system 12 are then triggered to switch on for example by a corresponding signal supplied by the control unit 18.

Due to the standby mode, in the event of operating interruptions in the system, a quite considerable energy saving is made, in particular a quite considerable saving of heating energy and thus associated with it a considerable reduction in waste gas emissions and in particular CO2 emissions in the case of a gas-powered shrink tunnel 10.

As explained, in conventional systems, even during operating interruptions, i.e. where no packaging material groups 3 are moved through the shrink tunnel for the shrinking-on of their shrink film, the operating temperature of the shrink tunnel is kept at the normal target operating temperature, for example at the operating temperature of 200° C. This demands a heat output which, although due to the absence of product groups 3 being moved through the shrink tunnel is reduced compared with the heat output in normal undisturbed operation, does however mean a heat output which corresponds to around 40% to 50% of the total output of the shrink tunnel in normal undisturbed operation and lies for example in conventional shrink tunnels in the range of 40 KW to 45 KW.

In a concrete embodiment of the shrink tunnel 10, the target operating temperature of which is 200° C. in normal undisturbed operation, with a temperature drop ΔT of 50° C., i.e. to a reduced tunnel temperature of 150° C. in standby mode, a reduction in heat output to 5.4 KW was observed and with a temperature drop ΔT of 80° C., i.e. to a reduced tunnel temperature of 120° C., a reduction in the heat output to 4.6 KW was observed, whereby the times taken to heat up to the target operating temperature stood at 2.5 minutes for the temperature drop AT of 50° C. and 4 minutes for the temperature drop AT of 80° C.

Even taking account of the time taken after the triggering of standby mode for the temperature to fall from the target operating temperature to the reduced tunnel temperature of standby mode, this being for example 8 to 10 minutes, there is a considerable energy-saving potential from switching into standby mode. Thus, for example, the energy saving in a one-minute disruption or interruption in operation lies between 1.44 KWh and 5.76 KWh and in a 30-minute disruption or interruption at around 18 KWh, this including the heating energy needed for re-heating at the end of the operating interruption.

A considerable energy saving results furthermore from the switching-off of the cooling system or cooling blower for the at least one conveyor belt 13 and the cooling zone 17, and also from the switching-off of at least one blower for the hot air in each tunnel zone 14, whereby these blowers have for example an energy demand of around 3 KW.

The return of the system and the shrink tunnel to normal operating status, i.e. the production approval and the power-up of the shrink tunnel 10 in normal operating status after the deletion of the error messages occurs for example in the following stages:

-   -   Re-setting of the tunnel temperature of the individual tunnel         zones 14 to the necessary target operating temperature set in         the control unit 18 and/or saved there, and switching-on of all         heating means 15 or all heating units and hot-air blowers;     -   Re-setting of the transport speed of the at least one conveyor         belt 13 to product speed and switching-on of the conveyor belt         cooling system for the at least one conveyor belt 13 when the         tunnel temperature in all tunnel zones 14 has reached a         temperature threshold during re-heating, which is around 5% to         7%, for example 5% or 10° C. below the target operating         temperature;     -   Release or switching-on of the packaging machine 7 and         switching-on of the cooling system for the packaging units or         containers 4, i.e. production approval once, during heating, the         operating temperature in all tunnel zones 14 has reached a         temperature threshold which is slightly, i.e. for example 2 to         3%, e.g. 5° C. below the target operating temperature.

The operating parameters characterising the normal operating mode and those characterising the standby mode, in particular the target operating temperature, the temperature drop ΔT and/or the reduced tunnel temperature, the time delays Δt1 and Δt2, the reduction in the speed of transport of the transport system on the shrink tunnel side or the at least one conveyor belt 13 etc. are adjustable on the control unit 18 as control or adjustment parameters and/or are saved there in a data memory.

In a preferred embodiment of the invention, for the shrink tunnel 10 at least one further standby mode or standby operation is provided, which is triggered by other criteria or parameters and/or has other measures than the standby mode described above which is triggered in the event of operating interruptions in the system. This additional standby mode can for example be provided for planned production interruptions in particular a known frequency of a preferably longer duration and/or with a planned start and end time, for example for planned or specified production breaks. As the end of this kind of planned production interruption, for example the end of a daily recurring break, is known and thus the time at which production is resumed, there can be, in the event of a planned or set production interruption of the shrink tunnel 10, a switch into a deeper standby mode, i.e. the tunnel temperature in the tunnel zones 14 can be lowered even further in comparison to the standby mode triggered by an operating interruption, as the restart-up too, in particular the re-heating-up of the tunnel zones can be planned and time-controlled.

The invention was described above using an example of an embodiment. It is clear that numerous modifications and variations are possible without thereby departing from the invention idea on which the invention is based.

REFERENCE DRAWING LIST

-   1 System section -   2 Packaging materials -   3 Packaging material group -   4 Packaging unit or container -   5 Buffer or storage unit -   6 External conveyor -   7 Packaging machine -   8 Packaging element -   9 Transport system for feeding the packaging elements 8 -   10 Shrink tunnel -   11 External conveyor -   12 Transport system -   13 Conveyor belt -   14 Tunnel zone -   15 Heating means -   16 Lamellar curtain -   17 Cooling zone -   18 Electronic control unit -   A, B Direction of transport -   AT Temperature drop in standby mode -   At1 Time delay at the triggering of the standby mode -   At2 Time delay at the re-heating of the shrink tunnel 

1-19. (canceled)
 20. An apparatus comprising a shrink tunnel for use in a production system for the production of packaging units from packaging materials filled with a product and having shrink film shrunk thereon, said shrink tunnel comprising a tunnel zone, heating means for application of heat to shrink said shrink film onto packaging units moved through said tunnel zone, said tunnel zone being maintained at a tunnel temperature equal to a target operating temperature, wherein said tunnel has an operating mode and a standby mode, wherein heat output during said standby mode is less than heat output during said operating mode, and wherein said standby mode is triggered by at least one of a time control and a signal control.
 21. The apparatus of claim 20, further comprising a control unit for controlling adjustment of said tunnel temperature via an output of said heating means such that during said normal operating mode, said tunnel temperature reaches said target operating temperature, and in said standby mode, said tunnel temperature is lowered to a reduced tunnel temperature to below said target operating temperature.
 22. The apparatus of claim 21, wherein said shrink tunnel comprises consecutive tunnel zones in a transport direction, and heating means allocated to each of said tunnel zones, each of said heating means being controlled independently for switching between said operating mode and said standby mode based on control parameters in said control unit.
 23. The apparatus of claim 21, wherein said control unit is configured to trigger said standby mode after a time delay.
 24. The apparatus of claim 21, wherein said control unit is configured to reduce transport speed of a conveyor on a shrink tunnel side after triggering of standby mode.
 25. The apparatus of claim 21, wherein said controller is configured to switch off a cooling system on a transport element on a shrink tunnel side after triggering of standby mode.
 26. The apparatus of claim 21, wherein said controller is configured to switch off a cooling system that serves to cool said packaging units after shrinking-on of said shrink film after triggering of standby mode.
 27. The apparatus of claim 21, wherein said controller is configured to switch off blowers of said heating means after triggering said standby mode.
 28. The apparatus of claim 20, wherein said control unit is configured to, after one of detecting disappearance of an error message and returning said shrink tunnel to normal operating mode, cause return of tunnel temperature to said target operating temperature and to switch on blowers of said heating means.
 29. The apparatus of claim 20, wherein said control unit is configured such that when a first temperature threshold of said tunnel temperature below said target operating temperature is reached, said control unit once again causes an increase in transport speed of said conveyor on said shrink tunnel side.
 30. The apparatus of claim 21, wherein said control unit is configured to generate a signal triggering production approval when a second temperature threshold closer to said target operating temperature than said first temperature threshold is reached.
 31. A method for operating a shrink tunnel for use in a production system for production of packaging units from packaging materials filled with a product and with shrink film shrunk thereon, said shrink tunnel including a tunnel zone, and heating means for application of heat to shrink said shrink film onto packaging units moved through said tunnel zone, said tunnel zone being maintained at a tunnel temperature equal to a target operating temperature, wherein said tunnel has an operating mode and a standby mode, wherein heat output during said standby mode is less than heat output during said operating mode, said method comprising, in response to an operating interruption of said shrink tunnel, triggering said standby mode with at least one of a time control and a signal control.
 32. The method of claim 31, further comprising controlling said tunnel temperature via an output from said heating means such that during normal operating mode said tunnel temperature corresponds to said target operating temperature and during standby mode said tunnel temperature to a reduced tunnel temperature.
 33. The method of claim 31, wherein controlling said shrink tunnel comprises controlling tunnel temperatures in consecutive tunnel zones in a transport direction, each of said tunnel zones having a specific heating means allocated thereto, said tunnel zones being controlled independently for switching between operating modes based on one of control parameters and adjustment parameters that are one of set in said control unit and stored in said control unit.
 34. The method of claim 31, further comprising controlling said heating means of said tunnel zone to operate between a heat output corresponding to a base load and a maximum heat output.
 35. The method of claim 31, further comprising triggering said standby mode in response to a signal indicating an operating interruption.
 36. The method of claim 31, further comprising returning said shrink tunnel to normal operating mode after disappearance of an error message.
 37. The method of claim 31, further comprising returning said tunnel temperature to target operating temperature and switching on blowers of said heating means when at least one of an error message disappears and said shrink tunnel is returned to operating mode.
 38. The method of claim 31, further comprising increasing transport speed of a conveyor on said shrink tunnel when a first temperature threshold of the tunnel temperature below the target operating temperature is reached.
 39. The method of claim 31, further comprising switching on a cooling system for said packaging units. 